Plate scale
The plate scale of a telescope connects the angular separation of an object with the linear separation of its image at the focal plane
![](../I/m/Plate_Scale.gif)
If focal length is measured in mm, the plate scale in radians per mm is given by angular separation θ and the linear separation of the image at the focal plane s, or by simply the focal length f:
since
Plate scale is usually expressed in arc-seconds per mm:
where f is in mm,[1] or expressed in arc-seconds per pixel after further division through the pixel scale.
Plate scale on JWST FGS/NIRISS
The plate scale of the James Webb Space Telescope component Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph is about 0.065 arcsec/pixel.[2] It uses a 2048 x 2048 pixel array with a pixel size of 18 microns a side with a field of view of 2.2' x 2.2' [3]